1. Field of the Invention
This invention relates to a method and apparatus of cementing casing in a wellbore and normally in a wellbore in which the casing will subsequently be heated to a higher temperature, causing it to have a tendency to elongate. It particularly concerns a method whereby the casing can be elongated without applying an upward mechanical force at the surface.
2. Setting of the Invention
In the search for oil and gas, boreholes are drilled deep into the earth. These holes are lined with casing, which is usually heavy steel pipe, and cement is forced between the casing and the borehole wall. In most cases, during production of oil and gas, the temperature of the casing doesn't vary much from what it is when it is originally set. However, in a growing number of situations, the fluid flowing through the wellbore is of such a high temperature that the casing is heated to a much higher temperature from that which it was when the casing was set. A thermal well can be a well in which steam or other hot fluid is injected down through a tubing string, suspended in the wellbore to aid in the recovery of fluid from the underground formation, or it can be a well which produces fluid from a formation which has a very high temperature. The increased temperature causes elongating stresses to be setup in the casing. It has been found that if the casing is hung and cemented and then large temperature differences are added to the casing, the tensile stress change for the fixed cementing casing is approximately 200 psi per degree Fahrenheit change.
In conventionally cementing a casing string in a wellbore, the casing string is preferably reciprocated and rotated during the placing or circulation of the cement between the outer wall of the casing and the wellbore. However, frequently, the reciprocation and rotation are dispensed with although it is normally desired if possible. The present invention permits conventional placing of cement, with or without reciprocation and rotation, and then placing the casing at extra tension while the cement sets. The extra-tensile stressing of the casing is then retained after the cement sets. This extra tensile stressing prevents heat from causing destructive compressive stresses in the casing once hot fluids are passed therethrough.
Instability in a casing string can be defined as lateral deflection of the casing due to buckling. The detrimental effects of instability in casing strings have long been recognized. These effects include not only damage to the casing string through excessive deformations, but also casing wear due to movement of drilling or production equipment through a buckled string of casing. Casing buckling can occur both above the cement top and in washouts below the cement top. At the present time, there exist three recognized procedures for increasing the stability of a casing string:
(1) Adjust pull/slackoff. PA1 (2) Adjust cement height. PA1 (3) Apply internal surface pressure during WOC (Waiting on Cement to set) time.
All three of the methods listed above will increase the stability of a casing string above the cement top. Only the last method will increase the stability of a casing string in a washout below the cement top.
Unfortunately, the three methods of increasing casing stability listed in the previous paragraph are not always easily applied in the field. Increasing the stability of a casing string by physically pulling on the string at the surface is a difficult job requiring jacking mechanisms. The foundation in the vicinity of the wellhead may not always be sufficient to support such an operation. Adjusting cement height is a fairly uncomplicated procedure. However, experience has shown that, especially in deep or wildcat wells, one cannot always be assured that his cement level will reach the anticipated position. The last method of increasing stability, application of internal surface pressure while waiting for the cement to take its initial set, is in many cases totally unsatisfactory. Application of internal pressure not only applies a tension force to the bottom of the string, but it also expands the casing radially. Once the cement has set and the internal surface pressure has been released, one is left with a microannulus between the casing string and the protective cement sheath. In operations where isolation of zones is of primary concern, this microannulus cannot be tolerated. The practice of the present invention prevents the formation of such microannulus.